Solar Energy Glossary S-Z


sacrificial anode — A piece of metal buried near a structure that is to be protected from corrosion. The metal of the sacrificial anode is intended to corrode and reduce the corrosion of the protected structure.

satellite power system (SPS) — Concept for providing large amounts of electricity for use on the Earth from one or more satellites in geosynchronous Earth orbit. A very large array of solar cells on each satellite would provide electricity, which would be converted to microwave energy and beamed to a receiving antenna on the ground. There, it would be reconverted into electricity and distributed the same as any other centrally generated power, through a grid.

scheduling — The general practice of ensuring that a generator is committed and available when needed. It also can refer to scheduling of imports or exports of energy into or out of a balancing area.

Schottky barrier — A cell barrier established as the interface between a semiconductor, such as silicon, and a sheet of metal.

scribing — The cutting of a grid pattern of grooves in a semiconductor material, generally for the purpose of making interconnections.

sealed battery — A battery with a captive electrolyte and a resealing vent cap, also called a valve-regulated battery. Electrolyte cannot be added.

seasonal depth of discharge — An adjustment factor used in some system sizing procedures which "allows" the battery to be gradually discharged over a 30-90 day period of poor solar insolation. This factor results in a slightly smaller photovoltaic array.

secondary battery — A battery that can be recharged.

self-discharge — The rate at which a battery, without a load, will lose its charge.

semiconductor — Any material that has a limited capacity for conducting an electric current. Certain semiconductors, including silicon, gallium arsenide, copper indium diselenide, and cadmium telluride, are uniquely suited to the photovoltaic conversion process.

semicrystallineSee multicrystalline.

series connection — A way of joining photovoltaic cells by connecting positive leads to negative leads; such a configuration increases the voltage.

series controller — A charge controller that interrupts the charging current by open-circuiting the photovoltaic (PV) array. The control element is in series with the PV array and battery.

series regulator — Type of battery charge regulator where the charging current is controlled by a switch connected in series with the photovoltaic module or array.

series resistance — Parasitic resistance to current flow in a cell due to mechanisms such as resistance from the bulk of the semiconductor material, metallic contacts, and interconnections.

shallow-cycle battery — A battery with small plates that cannot withstand many discharges to a low state-of-charge.

shelf life of batteries — The length of time, under specified conditions, that a battery can be stored so that it keeps its guaranteed capacity.

short-circuit current (Isc) — The current flowing freely through an external circuit that has no load or resistance; the maximum current possible.

shunt controller — A charge controller that redirects or shunts the charging current away from the battery. The controller requires a large heat sink to dissipate the current from the short-circuited photovoltaic array. Most shunt controllers are for smaller systems producing 30 amperes or less.

shunt regulator — Type of a battery charge regulator where the charging current is controlled by a switch connected in parallel with the photovoltaic (PV) generator. Shorting the PV generator prevents overcharging of the battery.

Siemens process — A commercial method of making purified silicon.

silicon (Si) — A semi-metallic chemical element that makes an excellent semiconductor material for photovoltaic devices. It crystallizes in face-centered cubic lattice like a diamond. It's commonly found in sand and quartz (as the oxide).

sine wave — A waveform corresponding to a single-frequency periodic oscillation that can be mathematically represented as a function of amplitude versus angle in which the value of the curve at any point is equal to the sine of that angle.

sine wave inverter — An inverter that produces utility-quality, sine wave power forms.

single-crystal material — A material that is composed of a single crystal or a few large crystals.

single-crystal silicon — Material with a single crystalline formation. Many photovoltaic cells are made from single-crystal silicon.

single-stage controller — A charge controller that redirects all charging current as the battery nears full state-of-charge.

smart grid — An intelligent electric power system that regulates the two-way flow of electricity and information between power plants and consumers to control grid activity.

soft costs — Non-hardware costs related to PV systems, such as financing, permitting, installation, interconnection, and inspection.

solar cellSee photovoltaic (PV) cell.

solar constant — The average amount of solar radiation that reaches the earth's upper atmosphere on a surface perpendicular to the sun's rays; equal to 1353 watts per square meter or 492 Btu per square foot.

solar cooling — The use of solar thermal energy or solar electricity to power a cooling appliance. Photovoltaic systems can power evaporative coolers ("swamp" coolers), heat-pumps, and air conditioners.

solar energy — Electromagnetic energy transmitted from the sun (solar radiation). The amount that reaches the earth is equal to one billionth of total solar energy generated, or the equivalent of about 420 trillion kilowatt-hours.

solar-grade silicon — Intermediate-grade silicon used in the manufacture of solar cells. Less expensive than electronic-grade silicon.

solar insolationSee insolation.

solar irradianceSee irradiance.

solar noon — The time of the day, at a specific location, when the sun reaches its highest, apparent point in the sky.

solar panelSee photovoltaic (PV) panel.

solar resource — The amount of solar insolation a site receives, usually measured in kWh/m2/day, which is equivalent to the number of peak sun hours.

solar spectrum — The total distribution of electromagnetic radiation emanating from the sun. The different regions of the solar spectrum are described by their wavelength range. The visible region extends from about 390 to 780 nanometers (a nanometer is one billionth of one meter). About 99 percent of solar radiation is contained in a wavelength region from 300 nm (ultraviolet) to 3,000 nm (near-infrared). The combined radiation in the wavelength region from 280 nm to 4,000 nm is called the broadband, or total, solar radiation.

solar thermal electric systems — Solar energy conversion technologies that convert solar energy to electricity, by heating a working fluid to power a turbine that drives a generator. Examples of these systems include central receiver systems, parabolic dish, and solar trough.

space chargeSee cell barrier.

specific gravity — The ratio of the weight of the solution to the weight of an equal volume of water at a specified temperature. Used as an indicator of battery state-of-charge.

spinning reserve — Electric power plant or utility capacity on-line and running at low power in excess of actual load.

split-spectrum cell — A compound photovoltaic device in which sunlight is first divided into spectral regions by optical means. Each region is then directed to a different photovoltaic cell optimized for converting that portion of the spectrum into electricity. Such a device achieves significantly greater overall conversion of incident sunlight into electricity. See also mulitjunction device.

sputtering — A process used to apply photovoltaic semiconductor material to a substrate by a physical vapor deposition process where high-energy ions are used to bombard elemental sources of semiconductor material, which eject vapors of atoms that are then deposited in thin layers on a substrate.

square wave — A waveform that has only two states, (i.e., positive or negative). A square wave contains a large number of harmonics.

square wave inverter — A type of inverter that produces square wave output. It consists of a direct current source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off at a correct sequence, at a certain frequency.

Staebler-Wronski effect — The tendency of the sunlight to electricity conversion efficiency of amorphous silicon photovoltaic devices to degrade (drop) upon initial exposure to light.

stand-alone system — An autonomous or hybrid photovoltaic system not connected to a grid. May or may not have storage, but most stand-alone systems require batteries or some other form of storage.

standard reporting conditions (SRC) — A fixed set of conditions (including meteorological) to which the electrical performance data of a photovoltaic module are translated from the set of actual test conditions.

standard test conditions (STC) — Conditions under which a module is typically tested in a laboratory.

standby current — This is the amount of current (power) used by the inverter when no load is active (lost power). The efficiency of the inverter is lowest when the load demand is low.

stand-off mounting — Technique for mounting a photovoltaic array on a sloped roof, which involves mounting the modules a short distance above the pitched roof and tilting them to the optimum angle.

starved electrolyte cell — A battery containing little or no free fluid electrolyte.

state-of-charge (SOC) — The available capacity remaining in the battery, expressed as a percentage of the rated capacity.

storage battery — A device capable of transforming energy from electric to chemical form and vice versa. The reactions are almost completely reversible. During discharge, chemical energy is converted to electric energy and is consumed in an external circuit or apparatus.

stratification — A condition that occurs when the acid concentration varies from top to bottom in the battery electrolyte. Periodic, controlled charging at voltages that produce gassing will mix the electrolyte. See also equalization.

string — A number of photovoltaic modules or panels interconnected electrically in series to produce the operating voltage required by the load.

sub-hourly energy markets — Electricity markets that operate on time steps of 5 minutes. Approximately 60% of all electricity in the United States is currently traded in sub-hourly markets, running at 5-minute intervals so that maximum flexibility can be obtained from the generation fleet.

substrate — The physical material upon which a photovoltaic cell is applied.

subsystem — Any one of several components in a photovoltaic system (i.e., array, controller, batteries, inverter, load).

sulfation — A condition that afflicts unused and discharged batteries; large crystals of lead sulfate grow on the plate, instead of the usual tiny crystals, making the battery extremely difficult to recharge.

superconducting magnetic energy storage (SMES) — SMES technology uses the superconducting characteristics of low-temperature materials to produce intense magnetic fields to store energy. It has been proposed as a storage option to support large-scale use of photovoltaics as a means to smooth out fluctuations in power generation.

superconductivity — The abrupt and large increase in electrical conductivity exhibited by some metals as the temperature approaches absolute zero.

superstrate — The covering on the sunny side of a photovoltaic (PV) module, providing protection for the PV materials from impact and environmental degradation while allowing maximum transmission of the appropriate wavelengths of the solar spectrum.

surge capacity — The maximum power, usually 3-5 times the rated power, that can be provided over a short time.

system availability — The percentage of time (usually expressed in hours per year) when a photovoltaic system will be able to fully meet the load demand.

system operating voltage — The photovoltaic array output voltage under load. The system operating voltage is dependent on the load or batteries connected to the output terminals.

system storageSee battery capacity.

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tare loss — Loss caused by a charge controller. One minus tare loss, expressed as a percentage, is equal to the controller efficiency.

temperature compensation — A circuit that adjusts the charge controller activation points depending on battery temperature. This feature is recommended if the battery temperature is expected to vary more than ±5°C from ambient temperature.

temperature factors — It is common for three elements in photovoltaic system sizing to have distinct temperature corrections: a factor used to decrease battery capacity at cold temperatures; a factor used to decrease PV module voltage at high temperatures; and a factor used to decrease the current carrying capability of wire at high temperatures.

thermophotovoltaic cell (TPV) — A device where sunlight concentrated onto a absorber heats it to a high temperature, and the thermal radiation emitted by the absorber is used as the energy source for a photovoltaic cell that is designed to maximize conversion efficiency at the wavelength of the thermal radiation.

thick-crystalline materials — Semiconductor material, typically measuring from 200-400 microns thick, that is cut from ingots or ribbons.

thin film — A layer of semiconductor material, such as copper indium diselenide or gallium arsenide, a few microns or less in thickness, used to make photovoltaic cells.

thin film photovoltaic module — A photovoltaic module constructed with sequential layers of thin film semiconductor materials. See also amorphous silicon.

tilt angle — The angle at which a photovoltaic array is set to face the sun relative to a horizontal position. The tilt angle can be set or adjusted to maximize seasonal or annual energy collection.

tin oxide — A wide band-gap semiconductor similar to indium oxide; used in heterojunction solar cells or to make a transparent conductive film, called NESA glass when deposited on glass.

total AC load demand — The sum of the alternating current loads. This value is important when selecting an inverter.

total harmonic distortion — The measure of closeness in shape between a waveform and it's fundamental component.

total internal reflection — The trapping of light by refraction and reflection at critical angles inside a semiconductor device so that it cannot escape the device and must be eventually absorbed by the semiconductor.

tracking array — A photovoltaic (PV) array that follows the path of the sun to maximize the solar radiation incident on the PV surface. The two most common orientations are (1) one axis where the array tracks the sun east to west and (2) two-axis tracking where the array points directly at the sun at all times. Tracking arrays use both the direct and diffuse sunlight. Two-axis tracking arrays capture the maximum possible daily energy.

transformer — An electromagnetic device that changes the voltage of alternating current electricity.

transparent conducting oxide (TCO) — A doped metal oxide used to coat and improve the performance of optoelectronic devices such as photovoltaics and flat panel displays. Most TCO films are fabricated with polycrystalline or amorphous microstructures and are deposited on glass. The current industry-standard TCO is indium tin oxide. Indium is relatively rare and expensive, so research is ongoing to develop improved TCOs based on alternative materials.

tray cable (TC) - may be used for interconnecting balance-of-systems.

trickle charge — A charge at a low rate, balancing through self-discharge losses, to maintain a

tunneling — Quantum mechanical concept whereby an electron is found on the opposite side of an insulating barrier without having passed through or around the barrier.

cell or battery in a fully charged condition.

two-axis tracking — A photovoltaic array tracking system capable of rotating independently about two axes (e.g., vertical and horizontal).

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ultraviolet — Electromagnetic radiation in the wavelength range of 4 to 400 nanometers.

underground feeder (UF) — May be used for photovoltaic array wiring if sunlight resistant coating is specified; can be used for interconnecting balance-of-system components but not recommended for use within battery enclosures.

underground service entrance (USE) — May be used within battery enclosures and for interconnecting balance-of-systems.

uninterruptible power supply (UPS) — The designation of a power supply providing continuous uninterruptible service. The UPS will contain batteries.

utility-interactive inverter — An inverter that can function only when tied to the utility grid, and uses the prevailing line-voltage frequency on the utility line as a control parameter to ensure that the photovoltaic system's output is fully synchronized with the utility power.

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vacuum evaporation - The deposition of thin films of semiconductor material by the evaporation of elemental sources in a vacuum.

vacuum zero — The energy of an electron at rest in empty space; used as a reference level in energy band diagrams.

valence band — The highest energy band in a semiconductor that can be filled with electrons.

valence level energy/valence state — Energy content of an electron in orbit about an atomic nucleus. Also called bound state.

varistor — A voltage-dependent variable resistor. Normally used to protect sensitive equipment from power spikes or lightning strikes by shunting the energy to ground.

vented cell — A battery designed with a vent mechanism to expel gases generated during charging.

vertical multijunction (VMJ) cell — A compound cell made of different semiconductor materials in layers, one above the other. Sunlight entering the top passes through successive cell barriers, each of which converts a separate portion of the spectrum into electricity, thus achieving greater total conversion efficiency of the incident light. Also called a multiple junction cell. See also multijunction device and split-spectrum cell.

volt (V) — A unit of electrical force equal to that amount of electromotive force that will cause a steady current of one ampere to flow through a resistance of one ohm.

voltage — The amount of electromotive force, measured in volts, that exists between two points.

voltage at maximum power (Vmp) — The voltage at which maximum power is available from a photovoltaic module.

voltage protection — Many inverters have sensing circuits that will disconnect the unit from the battery if input voltage limits are exceeded.

voltage regulation — This indicates the variability in the output voltage. Some loads will not tolerate voltage variations greater than a few percent.

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wafer — A thin sheet of semiconductor (photovoltaic material) made by cutting it from a single crystal or ingot.

watt — The rate of energy transfer equivalent to one ampere under an electrical pressure of one volt. One watt equals 1/746 horsepower, or one joule per second. It is the product of voltage and current (amperage).

waveform — The shape of the phase power at a certain frequency and amplitude.

wet shelf life — The period of time that a charged battery, when filled with electrolyte, can remain unused before dropping below a specified level of performance.

window — A wide band gap material chosen for its transparency to light. Generally used as the top layer of a photovoltaic device, the window allows almost all of the light to reach the semiconductor layers beneath.

wire typesSee Article 300 of National Electric Code for more information.

work function — The energy difference between the Fermi level and vacuum zero. The minimum amount of energy it takes to remove an electron from a substance into the vacuum.

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zenith angle — the angle between the direction of interest (of the sun, for example) and the zenith (directly overhead).

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